This invention relates to a heat transferable sheet or a sheet to be heat transfer printed, and more particularly to a heat transferable sheet which is used in combination with a heat transfer printing sheet wherein heat printing is carried out in accordance with image information by means of thermal heads, a laser beam, or the like.
Heretofore, a heat sensitive color-producing paper has been primarily used in order to obtain an image in accordance with image information by means of thermal heads, a laser beam, or the like. In this heat sensitive color-producing paper, a colorless or pale-colored leuco dye (at room temperature) and a developer provided on a base paper are contacted by the application of heat to obtain a developed color image. Phenolic compounds, derivatives of zinc salicylate, rosins and the like are generally used as such a developer.
However, the heat sensitive color-producing paper as described above has a serious drawback in that its color disappears when the resulting developed color image is stored for a long period of time. Further, color printing is restricted to two colors, and thus it is impossible to obtain a color image having a continuous gradation.
On the other hand, a heat sensitive transfer printing sheet wherein a heat-fusing wax layer having a pigment dispersed therein is provided on a base paper has been recently used. When this heat sensitive transfer printing sheet is laminated with a paper to be heat transfer printed, and then heat printing is carried out from the back of the heat sensitive transfer printing sheet, the wax layer containing the pigment is transferred onto the heat transferable paper to obtain an image. According to this printing process, an image having durability can be obtained, and a multicolor image can be obtained by using a heat sensitive transfer printing paper containing three primary color pigments and printing it many times. However, it is impossible to obtain an image having an essentially continuous gradation as in a photograph.
In recent years, there has been a growing demand for a method and means for obtaining an image like a photograph directly from an electrical signal, and a variety of attempts have been made to meet this demand. One of such attempts provides a process wherein an image is projected onto a cathode-ray tube (CRT), and a photograph is taken with a silver salt film. However, when the silver salt film is an instant film, the running cost is high. When the silver salt film is a 35 mm film, the image cannot be instantly obtained because it is necessary to carry out a development treatment after the photographing. An impact ribbon process and an ink jet process have been proposed as further processes. In the former, the quality of the image is inferior. In the latter, it is difficult to simply obtain an image like a photograph because an image treatment is required.
In order to overcome such drawbacks, there has been proposed a process wherein a heat transfer printing sheet provided with a layer of sublimable disperse dyes having heat transferability is used in combination with a heat transferable sheet, and wherein the sublimable disperse dye is transferred onto the heat transferable sheet while it is controlled to obtain an image having a gradation as in a photograph. According to this process, an image having continuous gradation can be obtained from a television signal by a simple treatment. Moreover, the apparatus used in this process is not complicated and therefore is attracting much attention.
One example of prior art technology close to this process is a process for dry transfer calico printing polyester fibers. In this dry transfer calico printing process, dyes such as sublimable disperse dyes are dispersed or dissolved in a solution of synthetic resin to form a coating composition, which is applied onto tissue paper or the like in the form of a pattern and dried to form a heat transfer printing sheet, which is laminated with polyester fibers constituting sheets to be heat transfer printed thereby to form a laminated structure, which is then heated to cause the disperse dye to be transferred onto the polyester fibers, whereby an image is obtained.
However, even if such a heat transfer printing sheet and a polyester fiber, heat transferable sheet are laminated and then subjected to heat printing by means of thermal heads or the like, it is impossible to obtain a developed color image having a high density. While one reason for this is that the surface of the polyester fiber fabric is not sufficiently smooth, it is thought that the main reasons are as follows.
In a conventional dry transfer calico printing process or a wet transfer calico printing process, the transfer of the sublimable dye onto the polyester fiber fabric is carried out with ample heating time. In contrast, heating by means of thermal heads or the like is ordinarily extremely short, whereby the dye is not sufficiently transferred onto the fiber fabric. In the dry transfer calico printing process, the transfer of the dye is accomplished by heating for about one minute at a temperature of 200.degree. C., whereas the heating by means of thermal heads is short, i.e., of the order of several milliseconds at a temperature of 400.degree. C.
In order to overcome these problems and obtain an image having a sufficiently high density, the formation of the image-receiving layer of a heat transferable sheet with a resin having low glass transition point and yet having a high affinity for a dye such as a polyester resin (Vylon, supplied by Toyobo, K.K., Japan) has been considered. In this case, the dye can easily permeate through the image-receiving layer even with the heating energy of a thermal head, and there is the possibility that a high-density image can be obtained.
In the case of the heat transferable sheet of this type, however, if the heat transfer sheet and the heat transferable sheet, after being mated with each other and heated, are peeled, the heat transfer layer per se adheres to the image-receiving layer of the heat transferable sheet and thus is peeled to be transferred thereonto, whereby both the sheets will never be fit for use. Presumably, the reason for this is as follows.
(i) Polyethylene terephthalate (PET) is generally used as a base film in the heat transfer sheet, but there are few binders that can bind a transfer layer fast to the base film.
(ii) In order to obtain a high image density, it is necessary to use a resin having low glass transition point and softening point for the image-receiving layer of a heat transferable heat. In general, however, such a resin softens and becomes viscous when energy is applied by a thermal head.
As a result of our further research with due consideration for the above facts, we have found that all the drawbacks mentioned previously can be eliminated by using a heat transferable sheet having a specific constitution. On the basis of this finding, we have arrived at the present invention.